Identifying the distinct roles of dual dopants in stabilizing the platinum-nickel nanowire catalyst for durable fuel cell

Stabilizing active PtNi alloy catalyst toward oxygen reduction reaction is essential for fuel cell. Doping of specific metals is an empirical strategy, however, the atomistic insight into how dopant boosts the stability of PtNi catalyst still remains elusive. Here, with typical examples of Mo and Au dopants, we identify the distinct roles of Mo and Au in stabilizing PtNi nanowires catalysts. Specifically, due to the stronger interaction between atomic orbital for Ni-Mo and Pt-Au, the Mo dopant mainly suppresses the outward diffusion of Ni atoms while the Au dopant contributes to the stabilization of surface Pt overlayer. Inspired by this atomistic understanding, we rationally construct the PtNiMoAu nanowires by integrating the different functions of Mo and Au into one entity. Such catalyst assembled in fuel cell cathode thus presents both remarkable activity and durability, even surpassing the United States Department of Energy technical targets for 2025.

Detailed comments 1. Figure 1e is a very important figure in this work, but it is extremely difficult to read (even in full color).I'd recommend the changes in MA and ECSA be on separate plots.I'd also recommend adding a plot the retained MA/ECSA for each catalyst (perhaps add to SI as part of this data is in Table S4).This will make it easier for the reader to compare with other published literature.2. Likewise, a table that summarizes the beginning of life values for MA & ECSA from RDE would be extremely useful for the readers.3. The introduction nicely covers relevant literature alloying Pt with Mo as an electrocatalyst.However, the authors should also discus recent literature from Esfahani where Mo or Mo + Si has been doped into metal oxide support materials and found to also greatly enhance Pt-based fuel cell electrocatalyst stability (doi:10.1016/j.apcatb.2016.08.041, doi:10.1016/j.apcatb.2019.118272, doi:10.1016/j.apcatb.2020.118743).In particular, they noted the suppression of Pt particle size growth is greatly supressed in these doped metal oxides through what is often referred to as "strong metal support interaction" (SMSI), which can also inhibit the sintering and leaching of active metals.The SMSI is often a general term to account for fermi level interactions between metal oxide support and catalytic nanoparticles.While the SMSI is common for many different metal oxide supports for fuel cells, but the durability reported in 10.1016/j.apcatb.2020.118743for a dual doped support that contained Mo + another dopant (Si) is also impressive.Thus, this reviewer wonders if mechanism of stabilization for Mo-doped electrocatalysts reported by the authors could be similar for Mo-doped supports.A short discussion of how these results could also benefit doped support design could make this paper have a greater impact in the field.4. Page 11,line 298: Please add " , respectively" after 0.05 mgPt cm−2 and 0.10 mgPt cm−2 5. Durability testing: The authors have tested all of their catalysts using variations of the DoE "load cycle" protocols, with the highest UPL of 1.0V vs RHE.Have the authors studied the durability of these catalysts using higher UPLS like those in the DoE "start-up/shutdown" test (UPL of 1.5V vs RHE)?There is already a massive amount of data in this paper, so I do not expect the authors to perform new experiments.However, these higher potentials do accelerate the rate of Pt NP growth and agglomeration compared to the load cycle protocol (though the data is often convoluted by the high rates of carbon corrosion).Thus, it would also be useful if the authors discussed/hypothesized about the impact of performing this work at higher UPLs, particularly since the development of more corrosion stable carbon and non-carbon supports is a highly active area of study.6. Durability Testing: On page 14: the authors state that the upper potential limit (UPL) used in their durability tests is 1.0 V vs RHE.However, Table S4 states the UPL is 1.1 V vs RHE.One of these is incorrect (most likely the Table ) and needs to be fixed.
Reviewer #3 (Remarks to the Author): The manuscript by Gao et al. reports on the study of PtNiAuMo alloys as the catalysts for the oxygen reduction for the application in fuel cells and clarifies the role of Au and Mo dopants on the enhancement of the activity and stability of the multimetallic alloy.The system described in the work demonstrates the performance surpassing that of the benchmark Pt/C and PtNi catalysts.The manuscript is well-structured and can be recommended for publication in Nature Communications once the following concerns are addressed: 1) The results of DFT calculations for ORR on Pt(111) discussed on P. 7-8 and Fig. S13 are significantly different from the published data on Pt(111) slab: in a seminal study [Phys. Chem. Chem. Phys., 2008, 10, 3722], the overpotential for ORR on Pt(111) is 0.75 eV which is significantly lower than the values reported in this manuscript not only for Pt, but also for PtNi, PtNiMo, PtNiAu.Referee acknowledges that different calculation parameters will have an impact on the absolute energy values, yet such an inconsistency for the relatively simple and well-studied system like metallic platinum has to be carefully addressed to support the validity of the DFT data in the manuscript.
2) What is the reason for a decrease of the Pt2+ fraction in PtNiMoAu alloy as compared to PtNi (Fig. 3g)?Is it because a smaller fraction of Pt sites is located on the surface of PtNiMoAu?This is also likely the dominant reason for the lower intensity of the white line in Pt L-edge XAS spectrum, rather than the electronic effects from the neighboring atoms as discussed on P. 9. 3) Analysis of the in situ XANES data indicating that oxidation of Pt in PtNiMoAu occurs at higher potential than for PtNi should be supplemented with the analysis of CV data, which should demonstrate the anodic shift of Pt/Pt2+ redox potential.These changes of the formal redox potentials of platinum should be explained in terms of the inductive effect from Mo/Au.4) Fig. S16 is absolutely confusing and should be removed as it represents a fitting of the noise.Author reply: We genuinely thank this reviewer for his/her constructive comments, which greatly improve the manuscript.Specifically, we have made the following changes according to the comments.

Point-by-point response to reviewer comments
(1) We have complemented the MEA studies for all reference catalysts and systematically analyzed the catalyst durability under MEA measurements (Supplementary Figs. 26,28 and 29;Supplementary Table 6).Consistently, both the MA and durability of the catalysts obtained under MEA measurements show the same trend with those under RDE tests.About the MA, the PtNiMoAu NWs/C presents a 6.6-, 2.0-, 1.3-, and 1.7-fold enhancement in the beginningof-life (BOL) MA (0.93 A mg −1 Pt) relative to the Pt/C (0.14 A mg −1 Pt), PtNi NWs/C (0.46 A mg −1 Pt), PtNiMo NWs/C (0.70 A mg −1 Pt), and PtNiAu NWs/C (0.55 A mg −1 Pt) at 0.9 ViR-free (Fig. 6c and Supplementary Fig. 26).Besides, the PtNiMoAu NWs/C also shows the highest MEA durability, presenting both lowest loss in MA and voltage (@0.8A cm -2 ) when compared to other catalysts (Supplementary Figs. 26 and 29).The changes of ECSA and composition for PtNi, PtNiAu, PtNiMo, and PtNiMoAu catalysts during the MEA durability tests were also monitored, which could well rationalize the observed durability trend (Supplementary Figs.28   and 29; Supplementary Table 6).Because this work just focuses on the Pt-based catalyst, we did not evaluate the oxygen/proton transport resistance under MEA measurements, which is generally related with the thickness of the catalyst layer, ionomer content, et al. (10.1016/j.electacta.2019.135474; 10.1039/d1cy00882j).We believe that these self-consistent results could strongly support our main conclusion of this manuscript.The relevant content has been added to our revised manuscript (page 11, lines 9-11, 17-20, and 31; page 12, lines 1-5).
(2) We agree with the reviewer's viewpoint that the MA target of US DOE refers to the MEA test instead of RDE.As thus, we have corrected the relevant description in the revised manuscript (page 11, lines 20-23).
We hope the revised manuscript guided by these valuable comments would reassure the reviewer for the publication.

Reviewer #2
The authors report a study of dual doping PtNi alloy catalysts with combination of Mo and Au to understand their role in enhancing durability in a fuel cell.While the use of these dopants/alloying elements is not new, the detailed study is quite novel and the results will have a significant impact in the development of electrocatalysts for fuel cells and electrolyzers.Overall the paper is well written, and the experimental work seems to be very well done.The performance of their PtNiMoAu/C catalysts are quite impressive.The paper needs some modest improvement in the presentation of the data and discussion of related literature as detailed below.
Author reply: We genuinely thank this reviewer for his/her constructive and positive comments.
1. Figure 1e is a very important figure in this work, but it is extremely difficult to read (even in full color).I'd recommend the changes in MA and ECSA be on separate plots.I'd also recommend adding a plot the retained MA/ECSA for each catalyst (perhaps add to SI as part of this data is in Table S4).This will make it easier for the reader to compare with other published literature.
Author reply: We genuinely thank this reviewer for his/her careful reading of our manuscript.As suggested, we have plotted the graphs for the MA and ECSA, as well as the retained MA/ECSA for each catalyst.The relevant figures/content have been revised/added in our revised manuscript (Fig. 1e,f; Supplementary Fig. 7; page 5, lines 25 and 26).

Likewise, a table that summarizes the beginning of life values for MA & ECSA from RDE would be extremely useful for the readers.
Author reply: As suggested, the beginning of life values for MA & ECSA from RDE were summarized in the SI (Supplementary Table 2).

The introduction nicely covers relevant literature alloying Pt with Mo as an electrocatalyst.
However, the authors should also discus recent literature from Esfahani where Mo or Mo + Si has been doped into metal oxide support materials and found to also greatly enhance Pt-based fuel cell electrocatalyst stability (doi:10.1016/j.apcatb.2016.08.041, doi:10.1016/j.apcatb.2019.118272, doi:10.1016/j.apcatb.2020.118743).In particular, they noted the suppression of Pt particle size growth is greatly surpressed in these doped metal oxides through what is often referred to as "strong metal support interaction" (SMSI), which can also inhibit the sintering and leaching of active metals.The SMSI is often a general term to account for fermi level interactions between metal oxide support and catalytic nanoparticles.While the SMSI is common for many different metal oxide supports for fuel cells, but the durability reported in 10. 1016/j.apcatb.2020.118743for a dual doped support that contained Mo + another dopant (Si) is also impressive.Thus, this reviewer wonders if mechanism of stabilization for Mo-doped electrocatalysts reported by the authors could be similar for Mo-doped supports.A short discussion of how these results could also benefit doped support design could make this paper have a greater impact in the field.

Manuscript
Gao et al. reported the synthesis of Mo and Au codoped Ptni nanowires and their performance forcathodic oxygen reduction catalysts in PEMFCs.It has been widely reported that the third metal doping(typically, Au, Mo, Rh, et al.)  could improve the stability of Pt/C and PtNi/PtCo alloy catalysts.Although the authors got some new understandings on how Mo and Au dopants work in improving the stability, I do not think this work in important enough for publish in Nature Communications.One of the key conclusions of this work is that the co-doping of Mo and Au can greatly improve the durability of PtNi catalysts in a synergetic manner.But the authors did not prove this issue in MEA, which is more relevant to practical PEMFCs applications than RDE.The durability test of the PtNi, Au-PtNi, Mo-PtNi, and AuMo-PtNi catalysts in MEA is necessary to confirm the codoping effect.The changes of ECSA and Ni content of these catalysts during the MEA durability tests should be demonstrated.The related MA, oxygen/proton transport resistance and voltage loss should be analyzed.The authors claimed that "the MA of the PtNiMoAu NWs/C catalyst presents even 6.6-fold increasement compared to the value that was set in the 2025 targets by U.S. Department of Energy (DOE)".Such conclusion is misleading, as the MA target of US DOE refers to the MEA test instead of RDE.